Conventional turbomachinery couplings transmit torque flexibly across concentric gear meshings, i.e., via mating internal/external teeth. Misalignment of the gear mesh relationship during operation produces sliding between mating teeth which will cause wear and demands on lubrication. There has been introduced into the turbomachinery industry new industrial couplings which are known as flexible diaphragm couplings. While the overall configuration of these couplings introduces various advantages, they also add a further and a new constraint to the design of coupled machinery. This is that there is a limit on the maximum allowed axial displacement of the coupled shafts during operation. Conventional gear couplings are relatively insensitive to axial displacement because of their spline-like relationship, whereas the diaphragm couplings are sensitive to such misalignment and if overstretched or excessively compressed axially, may be subject to failure by fatigue in less than 10.sup.8 shaft revolutions. Certain shaft temperatures and relative coefficients of thermal expansion either must be assumed or calculated to determine the initial amount of lateral and axial offset required in coupled machinery in order to produce minimum axial (and also angular) displacement after achieving anticipated operating temperatures. It is necessary, however, for these couplings to function properly during transient deflections which are associated with startup, shutdowns, and upsets which cannot be calculated in a straightforward manner. Thus, it is important to have a continuous monitoring system of axial displacement in those situations where the axial displacement is known to be sensitive to process load changes or there is a lack of confidence in the ability to predict the expected values of displacement.
To provide the necessary onstream axial displacement monitoring, there have been provided various commercial designs, none of which have been fully satisfactory in offering a low cost system which is simple in design and has absolutely no shutdown risk in the event that the monitoring device fails, and has the capability of repeating measurements. Some of these devices are of extremely high cost and very complex, while others such as those which employ a Vernier scale manual monitoring system are unsatisfactory from a safety standpoint. Still other Vernier type deflection indicators are indistinguishable under stroboscopic light examination. Some systems like that of U.S. Pat. No. 3,828,562, only provide a general indication of an axial position of the shaft within the tube by means of special markings and do not provide for remote readout like the present invention. Exemplary of devices which employ optical electronics and are highly complex and expensive are those disclosed in U.S. Pat. Nos. 3,762,217 and 3,871,215 which are both for torque and speed sensing. U.S. Pat. No. 3,688,570 discloses an angular deflection meter which, while perhaps less costly, is substantially more susceptible to contamination by dirt and grime and is relatively complex to mount, which is also true of some of the other prior art devices.
Thus, it is apparent from the prior art that there is a need for a deflection indicator device and/or system for use in connection with couplings of the flexible diaphragm type, which avoids and overcomes these deficiencies.